In the anesthetic field, the control of pains such as postoperative pains, cancerous pains and the like has recently been carried out by epidural catheterization in which a very small amount of liquid drug such as an analgesic is continuously administered to a patient. However, symptoms vary with the patients and patients occasionally complain of a sudden pain even when performing continuous microdose administration of analgesics. In order to cope with such a critical moment, there has been developed a device that is used by a patient for administering a dose of an analgesic to himself or herself. One example of such a device is a patient-controlled analgesic delivery system disclosed in Japanese national publication 63-501195 corresponding to international publication No. W087/00758.
The above patient-controlled analgesic delivery device has, as shown in FIG. 9a, a chamber 90 defined by a raised plateau 98 of a back plate 86 and a flexible circular sheet 96. This chamber 90 is connected to a first conduit communicating with a medical container and a second conduit communicating with a catheter or the like. Arranged on the flexible sheet 96 is a floating plate 100 which limits the volume of the chamber 90. Above the floating plate 100, there is arranged a push button 84 which allows the patient to operate the device by himself or herself. The base portion of the push button 84 is pivoted at its base portion by a pin 116 and biased by a coil spring 124 mounted around the pin 116. When the patient pushes the push button 84, the chamber 90 is depressed by the floating plate 100, as illustrated in FIGS. 9b and 9c, thereby delivering the liquid drug to the body of the patient through the conduit. When the finger is removed from the push button 84, the button 84 is returned to its original state by the coil spring 124, as illustrated in FIG. 9d.
In the above device, however, the flexible sheet 96 is, as shown in FIG. 9d, not restored to its original state by itself even when the push button 84 is released. Additional liquid drug can be introduced into the chamber 90 only by the pressure of the liquid drug squeezed out of the medical container which is so designed as to send out the liquid drug from a balloon by the restoring force of the balloon. However, it takes a long time to fill up the chamber 90 since the medical container is designed for microdose administration of the liquid drug and thus has a small volume of content. Accordingly, it is difficult to administer an additional dose of the liquid drug. In addition, the above device is limited in dosage to the maximum volume of the reservoir which can not be selected optionally, thus making it difficult to administer the dosage of drug required for the patient in a critical moment. Another problem of the above patient-controlled analgesic delivery device is that it is complex in structure and thus high in manufacturing cost. Further, there is a fear of leakage of the liquid drug from the circumference of the flexible sheet 96, as shown in FIG. 9.
In order to eliminate the aforementioned defects of the conventional device, therefore, we have already proposed a self-administration device for liquid drugs (as disclosed in Unexamined Published Japanese Patent Application No. 8-308925), which comprises: a cylindrical casing having a drug inlet port and a drug outlet port formed at a closed end; a reservoir so housed in the casing as to involve the drug inlet port and the drug outlet port; and pushing means fitted in an open end of the casing, so that when the patient pushes the pushing means, the reservoir is pushed by the pushing means to inject a liquid drug, as reserved in the reservoir, out of the drug outlet port.
Although this device has improved the aforementioned defects of the prior art, the device has such a high conduit resistance that it requires a significant force for injecting the liquid drug when a capillary means such as an epidural catheter is connected for use to the device. Another defect is that it takes a considerable time period to inject the liquid drug completely so that a patient having a declined strength will find it difficult to continue the pushing action of the pushing means. If the pushing means is pushed by a force more than necessary, moreover, the liquid drug may be abruptly injected out of the leading end of the capillary means such as the epidural catheter, and the leading end may go out of position or an excessively high internal pressure may be created in the device or the like to cause the liquid drug to leak.